Testing Procedures; Insulation Resistance Testing (R); Test Current Measurement (I); Polarization Index Testing (Pi) - Megger BM25 Guia Del Usuario

OPERATION

Testing Procedures

Insulation Resistance Testing (R)

This test mode measures insulation resistance continuously at the selected
voltage. The highest digital readings obtainable are 500 GΩ at 500 V and
5 TΩ at 5000 V , above which the specified accuracy can be expected to
decline. However, the analogue indicator display functions to 1TΩ at all
voltages.
1. Using the L M Range keys move the right hand cursor to 'R'.
2. Accept the default time of 30 minutes, or using the L M Selector
keys move the left hand cursor to the
test duration using the L M Range keys. Maximum test time is 90
minutes.
3. Using the L M Selector keys move the left hand cursor to the
required test voltage.
4. Start the test by pressing the red test button for at least one second.
5. On completion, the final resistance measurement is displayed
sequentially with the corresponding leakage current and the
capacitance value, where available. This sequential display repeats
until a key is pressed or Auto switch-off operates. For more details
of final display information see Appendix 1.

Test Current measurement (I)

This test mode measures insulation continuously at the selected voltage,
but displays leakage current value instead of resistance. This enables even
higher resistances to be calculated. Using this method it is theoretically
possible to measure resistance up to 500 TΩ at 5000 V but it will be
necessary to perform an open circuit calibration test to establish
measurement circuit offset current and test lead leakage. (This is ±0,2 nA
at normal temperatures with new, clean test leads).
10
position and then set the
1. Using the L M Range keys move the right hand cursor to 'I', and
follow the same procedure as for Insulation Test 'R'.
2. On completion, the final current leakage measurement is displayed
sequentially with the resistance and the capacitance value, where
available. This sequential display repeats until a key is pressed or
Auto switch-off operates. For more details of final display
information see Appendix 1.
Polarization Index and Dielectric Absorption Ratio Testing
When diagnostic testing insulation it is useful to investigate the way in
which the insulation reading varies with the time the test voltage has been
applied. One way is to compare the insulation resistance reading at two
different times dividing the later reading by the earlier reading. The result
is known as the Dielectric Absorption ration (DAR).
The normal appliaction of this is when the reading at 10 minutes is
divided by the resistance reading after 1 minute. this ratio is known as the
Polaristation Index (PI) of the sample.
The BM25 is capable of carrying out the more generalised DAR test and
the PI test automatically.
The BM25 uses 3 time settings when testing.
T3 Represents the total test duration. After this time period the BM25
will switch off the test current and discharge the sample under test.
T2 Represents the time of the second measurement requires to
evaluate a polarisation index and which must not be greated than
T3.
T1 Represents the time of the first measurement required to evaluate a
polarisation index.